For many years, aircraft have included as standard equipment, backup power supplied for use in times of power outage in the form of air driven turbines or fans. These turbines or fans are lowered into the airstream, thereby making use of the relative speed of the aircraft through the ambient air to cause the turbine or fan blades to rotate. The rotation imparted to the blades is then used to drive electrical generating gear or provide hydraulic power to the aircraft hydraulic system, or both during a power outage.
Conventionally, these devices are known as ram air turbines and include governor mechanisms that provide a measure of control over the output rotational speed delivered from the blade to the electrical or hydraulic units corresponding to the most efficient operating speed of such units.
One example of such a unit is illustrated in U.S. Pat. No. 4,411,596 issued Oct. 25, 1983 to Chilman, entitled "Ram Air Turbine Control System" and assigned to the same Assignee of the instant application. The Chilman device is successful for its intended purpose. However, because it employs fly weights which are used during startup of the device in addition to counterweights for the turbine blades, the same is larger than desired. Needless to say, in aircraft usages, bulk and weight are always of concern as is cost. Clearly, it would be desirable to reduce each.
A primary reason for the configuration of the Chilman device is that it provides a mechanism that may be stored out of the airstream with its blades in a coarse pitch position. When the blades are in coarse pitch position, the achievable speed is minimal. Consequently, if the blades are stuck-in one pitch position, due to ice or corrosion, when the ram air turbine is deployed after the blades have been stored in the coarse pitch position, the unit will not overspeed and self-destruct as might occur if the blades were stuck in a fine pitch position.
In normal starting, when the ram air turbine blades increase in rotational speed, the blades are conventionally transitioned from coarse pitch to fine pitch by the fly weights which move an entire spring carriage assembly. After that is accomplished, the turbine accelerates to a speed in the normal governing range and the primary governor, a counterweight associated with the blades and a governor spring take over and regulate speed from that point on.
Another reason for storing the ram air turbine with its blades in a coarse pitch position is due to the fact of air resistance to deployment. If the blades are in a coarse pitch position, the ram air turbine is aerodynamically "cleaner" during the deployment process that would be the case if the blades were in a fine pitch position. As a consequence, a smaller, and thus lighter and lower cost actuator for moving the turbine to a deployed position may be utilized.
The present invention seeks to provide a ram air turbine possessing all of the advantages of those heretofore known which are stored in the coarse pitch position and yet which is of less bulk by reason of a lesser axial length, of lesser weight because of the reduced bulk, and of lowered cost because of reduced complexity.